1. Field of the Invention
This invention relates to gas actuated air springs in general, and more particularly to air springs having an enhanced stroke capability for use as a counterbalance device.
2. Summary of the Prior Art
Counterbalance devices are used in weight lifting operations to oppose the load due to gravity. For example, U.S. Pat. No. 4,600,095 of Jul. 15, 1986, the teachings of which are incorporated herein by reference, shows a mechanical press which has an elevated welding station. A lift mechanism is disposed below a conveyor passing beneath the welding station. The lift mechanism is designed to elevate a platen with a straight line vertical motion to place a workpiece carried to the station on the conveyor into contact with the welding guns in the welding station. In such operation, the lift mechanism used to move the platen to place the workpiece in contact with the welding guns must accelerate to a relatively high velocity and then decrease to a near zero velocity at the end of each multiple stage stroke so that the workpiece is gently engaged against the welding guns, and thereafter gently deposited back on the conveyor. The workpiece may be raised and lowered through a distance of approximately 24 inches at a relatively rapid rate of one to two seconds. A counterbalance is used to reduce, usually by 85 percent or more, the power required to rapidly move the load which, without a counterbalance, would require substantially increased energy. One form of counterbalance frequently utilized is an air spring, which is comprised of a flexible rubber/fabric bellows having a column of compressed air within the bellows. The air spring provides a source of potential energy which may be utilized to counteract gravity acting on the platen. The platen may be moved toward and away from the work station, for example toward and away from the welding guns, by a lift mechanism such as that described in the aforementioned U.S. Pat. No. 4,600,095.
Air springs which have found use in such applications are manufactured, for example, by Firestone Industrial Products Co., of Noblesville, Ind. Typical commercially available air springs have a useful stroke of approximately three to eighteen inches. When an extended stroke is required, the air springs must be stacked on top of one another to increase the stroke without degrading their weight carrying ability. However, the foregoing arrangement requires that the stacked air springs be joined together by a center plate, for example, in the manner shown on page sixteen of the Firestone catalog number DKM-86A or in U.S. Pat. No. 4,825,681. Additionally, the plates connecting the air springs may be required to be guided by the use of vertical bars or rails, to insure accurate vertical motion of the plates connecting the air springs together. While the guidance rods used in stacked air spring applications help maintain stability, they require additional components and accurate alignment to insure that the stroke provided by the stacked air springs operate in a vertical column. Additionally, in some applications, it is impractical to use vertical guide bars due to space limitations. Finally, stacked air springs may simply not provide a relatively constant force over an extended travel distance required for some industrial applications.
Another possible way of increasing the stroke of an air spring is to increase the vertical height of the rubber/fabric bellows. However, bellows having substantially larger vertical dimensions can become slender and unstable with respect to its diameter, thereby risking catastrophic failure due to buckling. Also, increasing the vertical height of the rubber/fabric bellows often exceeds the capacities of existing forming equipment. New equipment to extend capacities is prohibitively expensive for all but the highest volume applications. Consequently, there is a need to increase the stroke of an air spring or counterbalance actuator while not adversely affecting the costs or mechanical stability.